1. Field to the Invention
The present invention relates to a golf club shaft and more particularly to a golf club shaft, made of a fiber reinforced resin, which is lightweight and has a high strength owing to improvement of a shock resistance at a tip side thereof.
2. Description of the Related Art
In recent years, the art for making a large head and a long shaft has been developed to hit a golf ball a long distance. However, it is difficult to swing the golf club having a large head mounted thereon. To facilitate swing of the golf club, it is necessary to make the golf club lightweight. To do so, there are growing demands for working out a design of reducing the weight of each part of the golf club.
As described above, it is necessary to work out the design of reducing the weight of each part of the golf club including the shaft. More specifically, in recent years, it has become possible to keep the rigidity of the shaft and make the shaft lightweight by composing the shaft of carbon fiber reinforced prepreg sheets whose resin content is not more than 25% or by composing the shaft of prepreg sheets having a high elasticity.
For example, in the golf club shaft disclosed in Japanese Patent Application Laid-Open No.5-49717, the straight layer (parallel with the axial direction of the shaft) has a two-layer construction consisting of the inner layer and the outer layer. The inner straight layer is made of carbon fibers having a high elasticity, whereas the outer straight layer is made of carbon fibers having a high strength. Thereby the shaft has a weight of not more than 63 g, supposing that the entire length of the shaft is 45 inches. That is, the proposed shaft is lightweight and has a proper degree of rigidity.
However, in the shaft disclosed in Japanese Patent Application Laid-Open No.5-49717, the strength of the high-elasticity carbon fibers is insufficient. Thereby the flexural strength of the shaft is affected by the high-elasticity carbon fibers of the inner layer having a low strength. In addition, the shaft has an insufficient strength in the compression direction. That is, the shaft having the above-described construction is incapable of having a sufficient strength. When a ball is hit, a great shock is applied to the neighborhood of the neck of the shaft where the head is mounted. The shaft does not have a sufficient strength against such a shock.
In the case where the carbon fiber reinforced prepreg sheet having a high elasticity is used for the shaft to keep the rigidity thereof, the breaking strength of the prepreg tends to fall. That is, as the tensile modulus of elasticity becomes higher from a peak of 290 GPa (about 30 ton/mm2), the breaking strength of the prepreg tends to fall. Accordingly the shaft using the high-elasticity carbon fiber reinforced prepreg sheet has a low strength. Therefore it is important for a lightweight shaft to have a sufficient strength. That being the case, a material (tensile modulus of elasticity is about 290 GPa), containing a small amount of carbon fibers, which provides the shaft with a sufficient strength effectively has been used by preference. However, there is a limitation in making the shaft which is lightweight and yet has a desired strength and rigidity.
As described above, when a ball is hit, a great shock is applied to the neighborhood of the neck of the shaft at its tip side where the head is mounted. That is, the neck is the most breakable portion of the shaft. The lightweight shaft is easy to swing and has a high head speed. Thus the lightweight shaft has a problem that the neck thereof is broken owing to application of a considerably great shock thereto. Therefore there are demands for development of a lightweight shaft and improvement of the strength of the shaft at its tip side.